One way of controlling the temperature of a comfort zone, such as a room in a building, is to control the flow rate of heated (or cooled) air supplied to the room. This can be accomplished by controlling the position of a supply air valve in response to an airflow sensor and a thermostat. The effectiveness of such an approach, however, can vary from one system to the next due to tolerance variations of the specific flow sensor being used.
A typical flow sensor provides an output voltage having an amplitude that varies as a function of airflow rate. The output of the sensor has a nominal null corresponding to zero flow and a nominal span corresponding to the expected range over which the output can vary as the flow rate varies from zero to full flow. Both the null and the span have a tolerance specified by the manufacturer of the sensor. Although some tolerance may be acceptable in some applications, it often causes serious control problems in the regulation of supply airflow to a comfort zone, especially at low airflow rates.
A flow sensor that senses airflow by sensing static and stagnation pressures (e.g. a Pitot tube in conjunction with a pressure-voltage transducer) typically provides an output voltage that has an exponential, rather than linear relationship with the rate of airflow. Problems arise at low airflow rates, because large changes in flow rate cause only small changes in output voltage. As a result, sensors having slightly different null values will generate significantly different voltage outputs at low flow rates.
Another problem with sensors having different null and span values is that the circuit receiving the sensor's output, typically an A/D converter, must be capable of receiving a wide range of signals. The range of the receiving circuit must be low enough to accept low airflow signals from sensors having a relatively small null value, and the range must be high enough for high airflow signals generated by sensors with a relatively high null and large span. As a result, any one particular sensor will use only a portion of the receiving circuit's full range. This means that the resolution will be less than optimum. Resolution, in a broad sense, is defined as the change in the receiving circuit's output for a given change in sensor output. Low resolution is especially detrimental at low airflow rates where a large change in airflow only causes a small change in sensor output.
Therefore, it is an object of the invention to provide a control for an air valve that compensates for the different characteristics of various airflow sensors without sacrificing resolution.
Another object of the invention is to adjust a flow sensor's output before it is delivered to a microcomputer's A/D converter so that various outputs from different sensors will closely fit within the range of the A/D converter.
A further object of the invention is to identify a valve or sensor malfunction by driving an air valve to a closed position and comparing the airflow sensor's output to a predetermined acceptable range.
A still further object of one embodiment of the invention is to accurately sense a flow sensor's null point by actually driving an air valve to a closed position rather than simulating a closed position by isolating the sensor from the airflow.
Yet another object of the invention is to provide high and low range look-up tables to represent the non-linear relationship between airflow and the flow sensor's output voltage and to provide a higher resolution during low flow rates.
These and other objects of the invention will be apparent from the attached drawings and the following description of the preferred embodiment.